1. Field of the Invention
This invention relates to photodetectors and in particular, photodetectors involving Group IV elements.
2. Art Background
Telecommunications systems include a light source such as a laser, a light guiding element such as an optical fiber, and a detector. (For purposes of this disclosure, a detector includes not only a photonic component that produces a signal in response to incident electromagnetic radiation, but also circuitry utilized in processing the resulting signal.) Significant research has been directed to finding a detector that allows the processing of signals at relatively high bit rates, e.g., bit rates greater than 100 Mbit/second, and at nominal cost. One well-investigated approach has involved germanium based photonic devices. For example, germanium p-i-n diodes--diodes having successive regions of p-type germanium, intrinsic material, and n-type germanium--constructed on a germanium substrate have been produced with electrical characteristics allowing light-stimulated signal production at bit rates of 100 Mbit/second and higher. (For purpose of this disclosure, intrinsic material is material, whether doped or not, that has a free majority carrier concentration of 5.times.10.sup.15 cm.sup.-3 or less.) Nevertheless, the cost of providing an interface between these germanium devices formed on a germanium substrate with suitable signal processing components (generally formed on a silicon substrate) has made the combination, i.e., the desired detector, economically unacceptable.
Germanium diodes have been formed on silicon substrates by, for example, depositing germanium of one carrier type on a silicon substrate of opposite carrier type. The result of this fabrication process has been totally discouraging. As described by K. Ito and K. Takahashi, Japanese Journal of Applied Physics, 7(8), page 821 (1968), germanium diodes formed by depositing germanium on a silicon substrate had a crystal dislocation average spacing of 90 Angstroms. (The average dislocation spacing is the average of the distances from a specific point on each crystal dislocation propagating entirely across the diode active region to a specific point on its nearest neighbor dislocation that also propagates across the active region. These points are defined by the intersection of such dislocations with an imaginary surface defined by the midpoints between the p-i interface and the i-n interface measured in a direction along a normal to the p-i interface.) This level of dislocation spacing in the active area of the diode leads to a very high rate of trapping for photogenerated carriers and results in a quantum efficiency that is intolerably low, e.g., well below 10 percent, and thus totally unacceptable for applications such as optical communication. Thus, substantial additional work concerning germanium based photonic components in a silicon environment has not materialized, possibly due to the apparent hopeless situation presented by initial investigations.
As an alternative to germanium based devices, photonic components based on III-V semiconductor materials have been developed. These components have excellent device characteristics. Although the interface cost for interconnection with suitable silicon based circuitry is still present, the quality of the device itself has, it appears, justified the concomitant expense. Nevertheless, for many applications, such as the high speed data links utilized with optical communication systems, it is still quite desirable to produce a detector that is more economic, even though less sensitive, than a III-V based photonic component with its associated silicon circuitry.